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 Post subject: Re: Atwater-Kent 20C 7960 - T4 audio transformer, open prima
PostPosted: Jan Sun 14, 2018 9:01 pm 
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How are the series resistors (about 600 ohms) on the grids of the first two tubes?
These often open, particularly the first one.

They're small wirewound resistors mounted on the back of the tuning capacitors.

You can repair them by putting a 1/20th watt resistor under the original and soldering its leads to the ends of the original part. These dissipate zero power, so that's not an issue.

- Leigh

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 Post subject: Re: Atwater-Kent 20C 7960 - T4 audio transformer, open prima
PostPosted: Jan Tue 16, 2018 9:44 pm 
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Posts: 23
OK

I've looked it over and I can see the bypass capacitor but it's not clear how to get to it.

It appears to be mounted somehow under the bakelite cover of the 2nd RF-amp tube pin-connectors.

I'm going to be VERY cautious when dealing with old brittle plastic; I want to
be sure what I am doing before I start.

I don't want to be the one to break a gem of a radio
because I'm a novice. :-)

Thanks

Ralph


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 Post subject: Re: Atwater-Kent 20C 7960 - T4 audio transformer, open prima
PostPosted: Jan Tue 16, 2018 9:55 pm 
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Joined: Dec Sat 09, 2017 11:58 pm
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OK

The series resistors (about 600 ohms) on the grids of the first two tubes show
no hint of dirt or corrosion and check out fine.

Thanks for you suggestion. Now I Know!

Movin On :-)


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 Post subject: Re: Atwater-Kent 20C 7960 - T4 audio transformer, open prima
PostPosted: Jan Tue 16, 2018 10:50 pm 
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RSC wrote:
OK

I've looked it over and I can see the bypass capacitor but it's not clear how to get to it.

It appears to be mounted somehow under the bakelite cover of the 2nd RF-amp tube pin-connectors.

I'm going to be VERY cautious when dealing with old brittle plastic; I want to
be sure what I am doing before I start.

I don't want to be the one to break a gem of a radio
because I'm a novice. :-)

Thanks

Ralph


Do you have the radio chassis out of the cabinet? As I recall, the RF bypass capacitor is only accessible with the chassis removed from the cabinet, as are the connections to just about everything else underneath the topside.

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 Post subject: Re: Atwater-Kent 20C 7960 - T4 audio transformer, open prima
PostPosted: Jan Wed 17, 2018 1:33 pm 
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Joined: Dec Sat 09, 2017 11:58 pm
Posts: 23
Yes,

I've removed the chassis from the cabinet and determined that the
grid leak resistor is open and the first audio transformer (larger can)
has an open primary. The final Transformer tests fine.

All other resistors and Caps test OK.

The radio is pristine and no hint of corrosion. Looks
like it came off the factory floor. :-)

------------

I can see how the transformer can be removed and replaced. No problem!

However, getting to the bypass capacitor and the bottom of the the grid leak resistor
seems to need some disassembly?

I wonder if I can get to the bypass capacitor it when I remove the dead transformer?

Thanks to all for your responses so far. I am making progress :)

Ralph


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 Post subject: Re: Atwater-Kent 20C 7960 - T4 audio transformer, open prima
PostPosted: Jan Wed 17, 2018 1:59 pm 
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Joined: Dec Sat 09, 2017 11:58 pm
Posts: 23
OK

On closer examination, I can see that I can solder the new grid leak resistor
on the bottom of the chassis (underneath) from the grid-pin of the detector tube
to the center-tap of the Detector Grid Bias resistor (180/270 Ohms).

Before I try this my concern is that the solder pad at the center-tap of the Detector Grid Bias resistor
is very small and looks fragile.

It ohms out just fine and I'd hate to damage an original part.

There is not enough room to solder a resistor directly under the original grid leak cartridge.


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 Post subject: Re: Atwater-Kent 20C 7960 - T4 audio transformer, open prima
PostPosted: Jan Wed 17, 2018 2:50 pm 
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For a grid leak resistor that snaps into a holder, there is another option to soldering in the replacement.

You can position the leads of a replacement resistor in the holder and snap the old resistor back in place to hold the leads of the replacement in place.

This assumes the original resistor measures high or is open.

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 Post subject: Re: Atwater-Kent 20C 7960 - T4 audio transformer, open prima
PostPosted: Jan Wed 17, 2018 4:33 pm 
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Location: S. Dartmouth MA 02748-1225 USA
Ralph,

Feel like getting creative with the A-K grid leak?

Unless the leak is made differently it has flanged metal ends and a smoked glass rod withing a glass tube.

The outer glass tube is held in place with solder and is considerably smaller than the solder filled socket in the metal end...

De-soldering using a soldering gun, forcing the metal end between the guns tips extension wires, current will flow through the metal and quickly heat the end without tinning the metal end. Use any other de-soldeering technique that works...

Tap the tip of the gun with the metal end to knock out the solder.

The larger socket is not quite large enough for a standard size of glass tubing so a twist drill, used by hand can ream out the remaining solder and a bit of the brass... A standard borosilicate glass tube will now fit into the socket.

Get a 1/10 watt 2.2meg any tube of resistor, a low noise metalized would be better. The diameter of the 1/10 watt resistor will fit within the glass tube. Measure the lead diameter of the resistor and drill the next size through the end caps.

Assemble the parts passing the resistor lead through the drilled end cap. Solder one end, assemble the other end and adjust for length, solder the end. A tiny drop of super glue will secure the glass tube to the metal ends.

If you don't like to see the color bands on the resistor, blacken it with a Sharpie...

YMMV

Chas


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 Post subject: Re: Atwater-Kent 20C 7960 - T4 audio transformer, open prima
PostPosted: Jan Thu 18, 2018 12:47 am 
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Joined: Dec Mon 02, 2013 12:24 am
Posts: 209
Location: Laplata, Maryland 20646
I have got them working,and quite well, by bypassing winding with a resistor and jumping primary to secondary with a cap, resistor allows voltage to plate, and cap passes signal to next stage , while blocking Dc' try it ! Google audio transformer bypassing, good luck and man is it fun when it comes to life.


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 Post subject: Re: Atwater-Kent 20C 7960 - T4 audio transformer, open prima
PostPosted: Jan Thu 18, 2018 1:10 pm 
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Joined: Dec Sat 09, 2017 11:58 pm
Posts: 23
OK!

One more question before I do the repairs.

----------

I've received the transformer from AES.

The secondary has a center-tap.

-----------

I believe that I've read elsewhere in this forum that I
should cut the center tap since it will be unused in this application.

Is that Correct?

-----------

Also, I can now see how the grid leak resistor can be soldered in.
Much easier than it initially appeared.

-----------

Thanks to all for your ideas and recommendations; greatly appreciated.

Ralph


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 Post subject: Re: Atwater-Kent 20C 7960 - T4 audio transformer, open prima
PostPosted: Jan Thu 18, 2018 1:38 pm 
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Correct. Center tap unused in this application.

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Paul
...... how hard can it be?


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 Post subject: Re: Atwater-Kent 20C 7960 - T4 audio transformer, open prima
PostPosted: Jan Wed 31, 2018 5:57 pm 
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Joined: Dec Sat 09, 2017 11:58 pm
Posts: 23
OK!

I'm back to repairing the radio after a long unplanned absence!

I have now successfully installed a new grid-leak resistor on the bottom of the chassis
without disturbing any other components. You would never see it unless you knew
what you were looking for ;-)

I have also identified how to remove the defective audio transformer but
haven't had the time to remove it.

I want to be absolutely certain that I have correctly identified and labeled
the transformer leads before removing it and don't have the time to be
sufficiently careful right now.

(I have the replacement transformer already.)

I'm hoping that the old freezer method of removing the old transformer
from the can works! :-)

I expect to get the transformer replaced over the weekend.

Then I'll recheck all the other components and the battery cables before connecting the ARBIII power supply.

I also have some extra tubes handy if I have a dead one (or two).

Thanks to all who have given their suggestions and support.

I'll be checking in soon.

All the best

Ralph C.


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 Post subject: Re: Atwater-Kent 20C 7960 - T4 audio transformer, open prima
PostPosted: Feb Sun 11, 2018 4:07 pm 
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Joined: Dec Sat 09, 2017 11:58 pm
Posts: 23
OK!

I have an update and a question to anyone who can help.

Before I removed the "defective" audio transformer with the open winding
I re-checked all of my continuity tests.

Incredibly I found that the 22V wire in the cable has an open (no continuity!)
I disconnected the 22V wire from the transformer and triple-checked my
measurement and it IS open!

(Good news is that measuring the transformer with the 22V wire disconnected
confirms the transformer is OK! Nice)

All the other wires in the cable read good continuity.

I've never seen a wire open like this!

Has anyone seen this before and how could this happen?

I have a new cable on order.

How is this possible?

Thanks for anyone who will comment.

all the best

Ralph C. (RSC)


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 Post subject: Re: Atwater-Kent 20C 7960 - T4 audio transformer, open prima
PostPosted: Feb Sun 11, 2018 7:21 pm 
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Some wiring can be prone to corrosion and will literally disintegrate inside the insulation with no external trace. Problems like that can make you pull your hair out because we all assume a wire is just a wire and should have electrical continuity. "Assume nothing", is the take-away.

In my experience, wire in 1920's radios seem more apt to do this than newer wire. Not sure if it was due to impurities in the copper, tinning, insulation or environmental factors?
Maybe someone repaired it using acid core solder in the past.

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...... how hard can it be?


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 Post subject: Re: Atwater-Kent 20C 7960 - T4 audio transformer, open prima
PostPosted: Feb Sun 11, 2018 9:19 pm 
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Joined: Dec Sat 09, 2017 11:58 pm
Posts: 23
Thanks for the prompt reply!

Of course I'll re-check all connections and component again before power-up

all the best

Ralph C.


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 Post subject: Re: Atwater-Kent 20C 7960 - T4 audio transformer, open prima
PostPosted: Mar Fri 02, 2018 1:26 pm 
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Joined: Dec Sat 09, 2017 11:58 pm
Posts: 23
OK!

Finally got back to the radio.

I bought a new power cable and replaced the old one.

After disecting the old cable I found that the 22 1/2 volt wire in the cable had a break.

It seemed to have been badly crimped and broke when flexed sometime
in the past.

I'll bet the radio broke soon after it was purchased and was put away
and replaced with a newer model :-)

I've replaced the power cable and re-checked connections, resistors, and capacitors-
all check out fine.

Next step is to install the tubes and power it up.

I'll be using the ARBE-III power supply.

I'd appreciate and recommendations and cautions before I apply power
and any operating tips suggestions.

I appreciate all of the comments and suggestions folks have given here on the forum!

all the best

Ralph


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 Post subject: Re: Atwater-Kent 20C 7960 - T4 audio transformer, open prima
PostPosted: Mar Fri 02, 2018 2:24 pm 
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Location: Gretna, Nebraska
You will like the power supply you chose. They are great.

When hooking up the power supply, you want to avoid accidentally applying B+ to the tube filaments which will instantly burn them out.

First remove the tubes, and then connect the power suppy to the radio. Apply power and measure the voltage at one of the empty tube sockets to confirm you have the correct voltages at the socket.

Plug in the tubes only after confirming you have the correct voltages.

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Paul
...... how hard can it be?


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 Post subject: Re: Atwater-Kent 20C 7960 - T4 audio transformer, open prima
PostPosted: Mar Fri 02, 2018 4:54 pm 
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Location: RI, 02885
RSC wrote:

Can you recommend a supplier for the cloth sleeve?


Look at shoelaces. Some will be large enough and can be stretched if needed. I've picked up some large ones for battery cables and dyed them black.

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 Post subject: Re: Atwater-Kent 20C 7960 - T4 audio transformer, open prima
PostPosted: Mar Tue 06, 2018 3:40 pm 
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Joined: Dec Sat 09, 2017 11:58 pm
Posts: 23
OK!!!

Great News!!!

After adding a grid leak resistor and replacing a defective power cable,
I FINALLY had the time to hook up the power supply, antenna, and speaker.

It Works!

It functions beautifully.

So fantastic to have a functioning piece of radio history.

I can't thank you enough, those who have offered advice to me during this project.

All the best!

Ralph Culver
rsc4home@aol.com


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 Post subject: Re: Atwater-Kent 20C 7960 - T4 audio transformer, open prima
PostPosted: Mar Tue 06, 2018 4:18 pm 
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Location: Gretna, Nebraska
Congratulations on your success!
Let the addiction begin.

I love the smell of hot rosin flux in the morning. It smells like....Victory!

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Paul
...... how hard can it be?


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